The manufacture of a semiconductor element or the like by photolithography uses a projection exposure apparatus for transferring a circuit pattern or the like formed on a master (to be referred to as a reticle hereinafter) such as a reticle or photomask to a semiconductor wafer or the like coated with a photosensitive agent. An exposure apparatus of this type must accurately transfer a pattern on a reticle to a wafer at a predetermined magnification (reduction ratio). To meet this demand, the exposure apparatus must exploit a projection optical system which exhibits good imaging performance and suppresses aberration. In recent years, a pattern exceeding the general imaging performance of an optical system is often transferred along with further miniaturization of a semiconductor device. The transfer pattern, therefore, is more sensitive to the aberration of the optical system than a conventional pattern. On the other hand, the projection optical system must increase the exposure area and numerical aperture (NA), which makes aberration correction more difficult.
In this situation, demands are arising from measuring imaging performance, e.g., aberration, particularly, wavefront aberration of the projection optical system while the projection optical system is mounted in the exposure apparatus, i.e., is actually used for exposure. This is because measurement of aberration enables more precise lens adjustment corresponding to the state and device design almost free from the influence of aberration.
To meet these demands, the image intensity distribution is measured by a knife edge or slit, as a conventional means of obtaining imaging performance while the projection optical system is mounted in the exposure apparatus. Alternatively, the contrast of a pattern having a specific shape such as a bar chart is obtained.
In the method of obtaining the image intensity distribution by a knife edge or slit, the S/N ratio of image intensity distribution measurement must be about 106 or more in a projection optical system used for semiconductor lithography. This value is difficult to achieve.
To obtain wavefront aberration in the method of obtaining the contrast by using a bar chart, the contrasts of many bar charts must be obtained from a rough pitch to a pitch exceeding the resolution limit. This is not practical in terms of the formation of bar charts and measurement labor.
Further, these methods do not allow measurement of wavefront aberration.
As a method of obtaining wavefront aberration, an interferometer is used. However, the interferometer is generally used as an inspection device in the manufacture of a projection optical system, and is not practically mounted in the exposure apparatus in terms of the technique and cost because an interferometer made up of a prism, mirror, lens, and the like, and an interference illumination system having good coherence must be arranged near a reticle stage or wafer stage in the method using the interferometer. In general, the space near the wafer stage or reticle stage is limited, and the sizes of the interferometer and illumination system must, therefore, be limited. Limitations are also imposed in terms of heat generating and vibration, and the interferometer is difficult to mount. With recent decreases in exposure wavelength, an interferometer light source having good coherence in the exposure wavelength region does not exist or is very expensive. Thus, it is not practical in terms of the technique and cost to mount an interferometer type aberration measurement device in a projection exposure apparatus.